1. Field of the Invention
The present invention relates to a three-phase bridge type power conversion device for converting a DC power from a battery or the like into a three-phase AC power so as to drive a rotating electric machine, or converting a three-phase AC power generated by a rotating electric machine into a DC power so as to supply the converted DC power to a DC power supply such as a battery. In particular, the present invention relates to a power conversion device including detection and protection means for a short circuit failure between terminals (i.e., an arm short circuit due to a short circuit breakdown or an improper turning on of a semiconductor switch, and a short circuit to power supply, a short circuit to ground or a short circuit between phases due to a mechanical contact, and the like).
2. Description of the Related Art
A conventional power conversion device usually includes a certain type of current detecting means for detecting excessive current caused by a short circuit failure (see, for example, Japanese Patent Application Laid-open No. 2001-275392). The excessive current detecting means described in Japanese Patent Application Laid-open No. 2001-275392 includes a three-phase bridge type power conversion circuit made up of a semiconductor switch and a low resistance resistor (shunt resistor) for detecting current that is inserted in a DC line on a low potential side of the power conversion circuit. Further, if a voltage drop across the resistor exceeds a predetermined value, it is decided that excessive current is flowing therethrough. Then, the semiconductor switch is turned off so as to protect the circuit against a short circuit failure.
However, the conventional technology has problems as follows.
Such a conventional power conversion device causes a power loss by the shunt resistor even in a normal state where a short circuit failure occurs. Therefore, an efficiency of the power conversion device is decreased. In addition, it is necessary to take heat release measures for preventing overheating due to the power loss. As a result, the power conversion device may have to be a large size, and a cost thereof may be increased. In addition, it is necessary to shut off a large current by the semiconductor switch after detecting the short circuit current flowing due to the short circuit failure. Therefore, a secondary breakdown of the semiconductor switch may occur deviating from a safe operation range thereof when the large current is shut off.